1. Field of the Invention
This invention relates to improvements in the optical head device of an optical information reproducing apparatus for optically reading out information recorded on a recording medium or an optical information recording-reproducing apparatus for optically recording and reproducing information on a recording medium, and particularly to an optical head device in which the light source unit is fixed and the movable portion comprising deflecting means and condensing means is movable.
2. Description of the Prior Art
Generally, in apparatuses for optically effecting recording and reproduction of information, it is desired that the time required for searching the information be as short as possible.
For example, in an optical disc apparatus, to realize high-speed random access, it is necessary to move the optical head unit radially of the disc at a high speed and for this purpose, attempts have been made to make the entire optical head unit compact and light in weight. However, if the output of the light source such as a semiconductor laser is increased to accomplish high-speed random access, a heavy heat sink will necessarily be required with a result that rather the access time becomes longer.
FIG. 1 of the accompanying drawings shows an optical head device according to the prior art. A linearly polarized light beam emitted from a light source 2 such as a laser is reflected by mirrors 4 and 6, is made into a divergent light beam by a lens 8 and enters a polarizing beam splitter 10. The linearly polarized light beam thus entering has its plane of vibration set in a direction in which it is transmitted through the polarizing beam splitter 10 and therefore, it is transmitted through the polarizing beam splitter 10, is made into a circularly polarized light by a phase plate 12 such as a quarter wavelength plate and is reflected by mirrors 14 and 16 which are deflecting means, whereafter it enters a condensing lens 18 which is condensing means, whereby it is condensed on an information recording portion 22 such as a pit on a substrate 20 such as an optical disc. The reflected light beam reflected by the information recording portion 22 travels back along the above-described optical path, and is made into a linearly polarized light having a plane of vibration orthogonal to that during the incidence, by the phase plate 12. This linearly polarized light cannot pas through the polarizing beam splitter 10 but is reflected by the polarizing beam splitter 10, and enters a photodetector 24, whereby the information on the substrate 20 is detected. The mirrors 14 and 16 are rotated for tracking and time axis correction to thereby effect light deflection.
In this example of the prior art, the access in the radial direction of the substrate 20 is effected by moving the rotary shaft 26 of the substrate 20 and therefore, high-speed access is impossible.
Thus, in order to realize high-speed access, it is desired to make the condensing lens 18 and the mirrors 14, 16 into a block movable radially of the substrate 20, and as a device which satisfies this desire, there has been devised an optical head device in which the light source and the photodetector are separate from the condensing lens system and only the condensing lens system is movable. In the condensing lens system of such an optical head device, a single mirror has been used to cause a light beam to enter the condensing lens.
FIGS. 2A and 2B of the accompanying drawings are enlarged views of the essential portions of a conventional optical head device using the above-mentioned single mirror. FIG. 2A shows a state in which a movable portion 30 comprising a mirror 28 and a condensing lens 18 is disposed at an appropriate angle and position, and FIG. 2B shows a state in which the movable portion 30 is inclined by an angle .theta. as compared with the state of FIG. 2A.
In FIG. 2A, the incident light beam 32 is reflected in an appropriate direction by the mirror 28 and becomes a light beam which enters a substrate 20 perpendicularly thereto, and the quantity of light of this light beam is limited by a stop 34, whereafter the light beam is condensed on the information recording portion 22 of the substrate 20, whereat it is reflected and then travels back along the original optical path. At this time, the reflected light beam reflected by the information recording portion 22 emerges from the stop 34 over the full effective diameter thereof and becomes a reflected light beam 36.
Now, when the movable portion 30 is inclined by an angle .theta.with respect to the vertical axis 38 of the substrate 20 as shown in FIG. 2B, the reflected light beam reflected by the information recording portion 22 travels in a direction forming an angle 2.theta. with respect to the vertical axis 38 of the recording medium 20 and at the same time, the optic axis of the condensing lens 18 also forms an angle .theta. with respect to said vertical axis 38. Therefore, the reflected light beam passes through a position laterally deviated from the stop 34 and thus, an unnegligible quantity of light is lost.
The light beam passed through the stop 34 emerges as a reflected light beam 36 through the condensing lens 18 and the mirror 28. As shown, the quantity of light of the reflected light beam 36 is smaller than the quantity of light of the incident light beam 32. In FIG. 2B, the reflected light beam 36 is depicted so as to be coaxial with the incident light beam 32, but this is only for the purpose of schematic representation and actually, the center of the optic axis of the incident light beam 32 is not coincident with the center of the optic axis of the reflected light beam 36, that is, the reflected light beam inclines relative to the incident light beam and therefore, the light beam detected by a photodetector (not shown) becomes deviated from its original appropriate position and thus, a detection error occurs.
Further, when the movable portion 30 is inclined by an angle .theta. with respect to the vertical axis 38 of the substrate 20, the light beam condensed on the information recording portin 22 also becomes deviated from the original position of the condensed light beam, whereby coma and astigmatism are liable to occur.
That is, the optical head device shown in FIGS. 2A and 2B has the following disadvantages which result from the deflecting means and the condensing means being inclined from their proper angles:
(1) The quantity of light of the reflected light is lost;
(2) Deviation occurs between the incident light and the reflected light and a detection error occurs; and
(3) Coma and astigmatism occur.